Necrotic Ooze and Phyrexian Altar on the battlefield.
Endling and Walking Ballista in your graveyard.
({B} magic symbol) available.
Steps
Activate Necrotic Ooze using Endling's third ability by paying ({B} magic symbol) , giving Necrotic Ooze undying until end of turn.
If Necrotic Ooze has any +1/+1 counters, activate it using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat step 2 until Necrotic Ooze has no +1/+1 counters.